1. Field of the Invention
The present invention relates to an ultra-small, waveguide, formed of a material having a high refractive index, for transmitting an optical signal, and to an optical coupling device employing the same, and more particularly, to a waveguide which can increase coupling by weakening a mode confine of a coupling area, and an optical coupling device employing the same
2. Description of the Related Art
In general, a waveguide consists of a core and a cladding layer formed of a material having a lower refractive index than that of the core. Accordingly, an optical signal is propagated in the waveguide by total internal reflection due to the difference in the refractive index between the core and cladding layer. The waveguide propagates only an optical signal satisfying a particular condition and the optical signal satisfying the propagation condition is referred to as a mode. The size of the mode is inversely proportional to the difference in the refractive index between the core and cladding layer.
Thus, as the difference in the refractive index between the core and cladding layer increases, the size of the mode decreases so that a waveguide having a small sectional area can be designed. Also, since loss in a bent area decreases, the bending radius can be made small. For example, when the core is formed of silicon Si having a refractive index of about 3.5 and the cladding layer is formed of silica SiO2 having a refractive index of about 1.5, the difference in the refractive index between the core and cladding layer is about 2.0, and an ultra-small waveguide having a sectional area in units of microns can be manufactured.
A silicon-based waveguide has a problem in that, when it is applied to an optical coupling device needing coupling such as a ring resonator filter, a waveguide filter, a directional coupler, and a waveguide modulator, coupling is weakened. That is, since the mode confined of the waveguide becomes excessive due to a wide difference in the refractive index between the core and the cladding layer or air, even when a gap between neighboring waveguides is maintained extremely narrow in units of microns, coupling is not smoothly performed. Also, since the waveguides contact each other, yield is deteriorated.
As an example of an optical coupling device to overcome the above coupling problem, a micro-resonator device configured as shown in FIG. 1 has been suggested. Referring to FIG. 1, the micro-resonator device includes a ring resonator 3 and first and second waveguides 1 and 5 arranged close to the ring resonator 3. The first waveguide 1 has a linear structure and an input port 1a and an output port 1b. The first waveguide 1 is separated from the ring resonator 3 with a predetermined gap in a tangential direction of the ring resonator 3. The second waveguide 5 is a curved type and includes an input portion 5a, a curved portion 5b, a linear portion 5c, and an output port 5d. The second waveguide 5 is separated from the ring resonator 3 with a predetermined gap in a tangential direction of the ring resonator 3.
In the micro-resonator device configured as above, when the ring resonator 3 operates, the first waveguide 1 and the ring resonator 3 are coupled so that an optical signal X1 input through the input port 1a passes through the ring resonator 3 and is input to the second waveguide 5. Thus, an optical signal X2 is output from the output port 5d of the second waveguide 5. When the ring resonator 3 is turned off, the optical signal X1 passes through the first waveguide 1 and an optical signal X3 is output from the output port 1b. In the micro-resonator device, a determinant of a coupling ratio is a gap between the first waveguide 1 and the ring resonator 3. For a high coupling ratio, the gap needs to be about 0.1 μm. However, the manufacturing process to maintain a narrow gap is complicated.